1. Field of the Invention
The present invention relates to a scrambling and unscrambling method for composite video signals, and to an implementing device.
1. Discussion of the Background
The methods currently used for scrambling video signals, particularly in the field of pay television, have the feature of, on the one hand, transmitting access messages, valid for all receivers, simultaneously with the video signals and, on the other hand, of periodically transmitting specific authorization messages to each receiver in a different way, for example by post or by telephone. Because of their method of transmission, the transmission periodicity of these authorization messages can only be relatively long, for example one month. A period as long as this can easily be taken advantage of by "pirates" who thus have sufficient time to discover the unscrambling key and to take advantage of it. Furthermore, this known method does not allow the paying of a subscription for only a portion of the programmes broadcast on a pay television channel if one is not interested in the totality of these programmes.